Conventionally, an aftertreatment device is incorporated in an exhaust pipe for a diesel engine to conduct exhaust emission control. Known as this kind of aftertreatment devices are a particulate filter which captures particulates (particulate matter) discharged from a diesel engine as well as a selective reduction catalyst with a property of selectively reacting NOx (nitrogen oxides) with a reducing agent even in the presence of oxygen.
The particulate filter comprises a filter body with a porous honeycomb structure made of ceramics such as cordierite and having lattice-like compartmentalized passages; alternate ones of the passages have plugged inlets and the remaining passages with unplugged open inlets have plugged outlets. Only the exhaust gas passing through thin porous walls compartmentalizing the respective passages is discharged downstream, and the particulates in the exhaust gas are captured by and accumulated on inner surfaces of the thin porous walls. Thus, the particulate filter is to be regenerated through burning and removal of the particulates before exhaust resistance is increased due to clogging by the particulates.
Meanwhile, well known in a field of industrial flue gas denitration in a plant or the like is effectiveness of using ammonia as a reducing agent for reduction and depuration of NOx. However, in a field of automobile where problematic is running with ammonia itself loaded, researches have been made nowadays on use of nontoxic urea water as a reducing agent. More specifically, addition of urea water into exhaust gas upstream of a selective reduction catalyst brings about thermal decomposition of the urea water into ammonia and carbon dioxide in the exhaust gas, leading to satisfactory reduction and depuration of NOx in the exhaust gas by ammonia on the selective reduction catalyst. Since urea water freezes at a temperature of −13.5° C. or less, it is necessary for a vehicle assumed to be used in a cold weather region to have provision for unfreezing of urea water having frozen in a urea water tank or midway of a urea water supply line. To this end, an coolant piping having engine coolant flowing therethrough is wrapped around an outer periphery of the urea water tank or of the urea water supply line and the coolant flowing through the coolant piping is enhanced in temperature to unfreeze the freezing urea water.
In order to regenerate the particulate filter as the aftertreatment device or unfreeze the urea water for the selective reduction catalyst as the aftertreatment device, an exhaust flow rate is throttled by exhaust throttle means during idling to rise a pressure of the exhaust gas upstream of the exhaust throttle means and thus rise an exhaust temperature. Further, the increase in exhaust resistance makes it difficult for relatively low-temperatured intake air to flow into cylinders of the engine and thus increases a remained amount of relatively high-temperatured exhaust gas; air in the cylinders including such relatively high-temperatured exhaust gas in much quantity, which is compressed in a next compression stroke and enters into an expansion stroke, attains further rising of the exhaust temperature and a warm-up operation of the diesel engine associated therewith rises the temperature of the coolant. The exhaust throttle means normally used is an exhaust brake incorporated in the exhaust pipe.
In this connection, if the exhaust brake as the exhaust throttle means were excessively closed, then load would become excessively high, resulting in failed rotation of the diesel engine; to the contrary, the exhaust brake, which were excessively opened, would fail in rising of the exhaust temperature. Therefore, it is very important to determine whether load increase by the exhaust brake as the exhaust throttle means is proper or not.
There exists, for example, Patent Literature 1 showing general state of the art pertinent to a device for determining whether load increase by an exhaust brake is proper or not.